The present invention relates to a catalyst support constituted essentially of a complex of magnesium chloride and n-butanol (n-BuOH). This complex is formed of particles with a high porous volume and has a low specific surface. It is prepared by precipitation of MgCl.sub.2 in solution in n-butanol at a temperature below the melting point of the complex from an emulsion of the said solution.
A catalyst for the polymerization of olefins can be obtained from the support by deposition under known conditions of a transition metal compound as well as other possible compounds which are customary with this type of catalyst.
According to British Patent No. 2,029,840 it is known to mix magnesium chloride, an alcohol or a phenol and a mineral oil and to form an emulsion of the system at a temperature of circa 120.degree. C. so as to maintain the magnesium chloride-alcohol or phenol complex in the molten state. After tempering the hot emulsion in a very cold nonsolvent of the complex, one recovers a product with a specific surface of 300 to 500 m.sup.2 /g, the particle porosity of which is between 0.3 and 0.4 cm.sup.3 /g.
It is also known from U.S. Pat. No. 4,499,194 to manufacture a support constituted of a complex of magnesium chloride and alcohol or phenol. According to this patent, magnesium chloride is made to react with an alcohol or a phenol in an inert organic solvent so as to form a complex, the system is then maintained in suspension in an inert organic solvent at a temperature between 85.degree. and 170.degree. C. so as to maintain the complex in the molten state. From this hot suspension are precipitated, by means of an organo-aluminic compound or a silicon halide, particles that have a specific surface between 200 and 700 m.sup.2 /g and a porosity between 0.2 and 0.7 cm.sup.3 /g.